I am sure you can Geoff good luck. I found with arches the wider the better for getting through with things like wheelbarrows etc. Also I have seen the posts put in to 'Metpost' and hammered in to stop rotting and so much digging out.

I've noticed in my local park that several of the arches have galvanised steel square section steel ( about 1 inch square ) set into concrete and protruding 6 inches out of the ground. The vertical wooded posts of the arch are then attached to these with a gap of about an inch between the bottom of the post and the ground.